DETERMINATION OF HETERONUCLEAR LONG-RANGE H,X COUPLING-CONSTANTS FROMGRADIENT-SELECTED HMBC SPECTRA

A gradient-selected J-HMBC experiment is presented. Coherence selectio n with magnetic field gradients results in HMBC spectra in which t(1) noise is very low or completely eliminated. This makes it possible to detect unequivocally very small (< 2 Hz) long-range H,X coupling const ants. With a slight modification, the HMBC experiment can be used for the quantitative measurement of these couplings. The signal amplitude of spectra with increasing coupling evolution time tau shows the chara cteristic sin (pi J(XH) tau) dependence. By fitting a sine curve to th e experimental data or by performing a 3D J-resolved HMBC experiment, accurate (n)J(XH) coupling constants are obtained. This method is espe cially useful for estimating coupling constants between protons and un protonated heteronuclei, e.g. quaternary carbons, N-15 nuclei in label led prolines or P-31 in organophosphates.